PROJECT SUMMARY An evolutionarily ancient strategy to activate innate immune defenses involves monitoring for changes in cellular physiology, rather than the presence of pathogens themselves. Key insights into these mechanisms have come from studies in the nematode C. elegans, which consume bacteria for food, and rely on ?surveillance immunity? to discriminate pathogens from potential food sources. Although several elegant examples of surveillance immunity have been described, the host proteins that sense intracellular pathogen- associated ligands to activate protective host responses are not known. As a critical first step to solving this problem, we have identified a cellular surveillance network that coordinates immune effector induction in C. elegans. These data offer a new paradigm of how immune pathways are activated in the intestinal epithelial cells of a metazoan host. The central hypothesis of this proposal is that a surveillance network composed of nuclear hormone receptors (NHRs) functions an intracellular monitor for small molecule toxins (or their effects) and engages protective immune responses. In support of this hypothesis, we have made several key observations that provide the rationale for the proposed work: (i) An immunostimulatory small molecule called R24 causes the robust induction of immune effectors via a mechanism that is distinct from known innate immune pathways, but is dependent on the conserved transcriptional regulator MDT-15/MED15. (ii) A single NHR is the central regulator in a cellular network that surveys intestinal epithelial cells for xenobiotic toxins. We have shown that this NHR drives the induction of protective defense responses by physically interacting with MDT-15. (iii) A specific fatty acid, whose synthesis is regulated by MDT-15, is required for the induction of immune defenses. In this proposal, we will characterize a surveillance network that activates immune defenses following toxin exposure (Aim 1). We will define the role of the fatty acid signal in controlling immune activation (Aim 2). We will determine whether the immune surveillance network acts locally within intestinal epithelial cells or systemically in mediating pathogen defense (Aim 3). Mechanisms of surveillance immunity are among the most primitive forms of pathogen detection in metazoans, and are conserved in humans. Thus, it is our expectation that insights from these studies in C. elegans will reveal evolutionary conserved strategies of immune activation, toxin recognition and cellular homeostasis in mammalian immunity.